A conventional image reader, which reads an image of a document sheet placed on a platen, detects an open or closed state of a document cover. The image reader performs a reading preparation operation based on the detected result before a reading start instruction is issued, thereby causing reduction of a time period from a timing when the reading start instruction is received to a timing when an actual reading operation is started.
One such a conventional image reader reads a reference white plate in response to a detected closed state of a document cover, and calculates data for shading correction on the basis of white data acquired using the read result.
Japanese Patent Application Publication No. 2001-77990 discloses another image reader that detects white data and black data in response to a detected open state of a document cover, and determines whether the detected black data and white data are influenced by light from outside of the image reader (hereinafter, referred to as ambient light). When there is no influence of the ambient light on the detected black data and the detected white data, the image reader reads an image of a document sheet by using the detected black data and the detected white data. When the detected black data and the detected white data are influenced by the ambient light, previous black data and previous white data, which were previously used on the basis of a previous reading start instruction, are used for current reading of the image of the document sheet.